The present invention relates generally to an absorbent composition for absorbent articles such as diapers, incontinence products, training pants, sanitary napkins, and the like. In particular, the present invention is directed to an absorbent composition having unexpectedly high fluid capacity and minimal gel-blocking properties, an absorbent article comprising said absorbent composition and a method of preparing the absorbent composition.
Absorbent materials that can absorb large amounts of liquids, such as water or body exudates, have many applications in disposable absorbent articles such as baby diapers, sanitary napkins, wound dressings, bandages, incontinent pads, and the like. Much effort has been expended to find cost-effective materials for absorbent articles which display good liquid absorbency and retention. Superabsorbent polymers (xe2x80x9cSAPsxe2x80x9d) in the form of granules, beads, fibers, bits of film, globules, etc. have been favored for such purposes. Such SAPs are generally water-insoluble and water-swellable substances capable of absorbing fluids in an amount which is at least ten times the weight of the substances in their dry form.
In one type of SAP, the particles or fibers may be described chemically as a crosslinked, sodium-neutralized polyacrylate. Included in this class of materials are such modified polymers as sodium-neutralized crosslinked polyacrylates and polysaccharides including, for example, cellulose, starch and regenerated cellulose which are modified to be carboxylated, phosphonoalkylated, sulphoxylated or phosphorylated, causing the SAP to be highly hydrophilic. Such modified polymers may also be crosslinked to reduce their water solubility.
The water absorption and water retention characteristics of SAPs are due to the presence in the polymer structure of ionizable functional groups. These ionizable functional groups are usually carboxyl groups, a high proportion of which are in the salt form when the polymer is dry but which undergo dissociation upon contact with water. In the dissociated state, the polymer chain will have a series of functional groups attached to it, wherein the groups have the same electric charge and thus repel one another. This leads to expansion of the polymer structure which, in turn, permits further absorption of water molecules although this expansion is subject to the constraints provided by the cross-links in the polymer structure which must be sufficient to prevent dissolution of the polymer.
The degree of cross-linking of superabsorbent polymers can be an important factor in establishing their absorbent capacity and gel strength. Absorbent polymers useful in absorbent articles generally need to have adequately high sorption capacity, as well as adequately high gel strength. Gel strength relates to the tendency of the swollen polymer to deform under an applied stress. If gel strength is low, the polymer can deform to such an extent so as to fill the capillary void spaces in the absorbent material to an unacceptable degree, thereby inhibiting the rate of fluid uptake or the fluid distribution by the absorbent material. Such gel deformation is generally referred to as xe2x80x9cgel-blocking.xe2x80x9d
Once gel-blocking occurs, further fluid uptake or distribution takes place primarily via a slow diffusion process. In practical terms, this means that gel-blocking can substantially impede the distribution of fluids to relatively dry zones or regions of the absorbent material. Thus, leakage from an absorbent article including the absorbent material can take place well before the particles of absorbent polymer in the absorbent material are fully saturated or before the fluid can diffuse or wick past the xe2x80x9cblockingxe2x80x9d polymer into the rest of the absorbent material.
In general, increasing gel strength will result in an increase in the permeability of an absorbent material comprising swollen absorbent polymer. However, this typically also reduces the absorbent capacity of the gel undesirably. See, for example, U.S. Pat. No. 4,654,039 to Brandt et al. and U.S. Pat. No. 4,834,735 to Alemany et al.
This gel-blocking phenomenon has typically necessitated the use of a fibrous matrix in which are dispersed the particles of absorbent polymer. This fibrous matrix keeps the particles of absorbent polymer separated from one another and provides a capillary structure that allows fluid to reach the absorbent polymer located in regions remote from the initial fluid discharge point. See U.S. Pat. No. 4,834,735 to Alemany et al.
As is apparent from the foregoing, each of the above references, presents a variety of means for preparing absorbent materials. However, all of these proposed means are deficient in terms of effectiveness and low product quality, mechanical complexity in design, and/or associated cost inefficiencies. For example, dispersing the absorbent polymer in a fibrous matrix at relatively low concentrations in order to minimize or avoid gel-blocking can significantly increase the bulkiness of the absorbent article or lower the overall fluid storage capacity of thinner absorbent structures.
In view of the deficiencies of the various products and processes disclosed in the above references, it is highly desirable to provide an absorbent composition that is superior in fluid capacity and gel-blocking properties. It is also highly desirable to provide an absorbent article comprising said absorbent composition and a method of preparing the absorbent composition.
In general, the present invention is directed to an absorbent composition that has an unexpectedly high fluid capacity, also referred to as absorptive capacity, and minimal gel-blocking properties, as well as absorbent articles comprising the absorbent composition and methods of preparing the absorbent composition. More particularly, the absorbent composition of the present invention has a Finite Volume Absorption Under Load (xe2x80x9cFVAULxe2x80x9d) value of at least about 60 cc after 10 minutes.
One embodiment of the present invention is an absorbent composition comprising: about 5% by weight to about 30% by weight of a fibrous matrix comprising wettable fibers and about 70% by weight to about 95% by weight of a surface crosslinked polymer having a FVAUL value of at least about 60 cc after 10 minutes, said surface crosslinked polymer being disbursed within said fibrous matrix.
A further embodiment of the present invention is an absorbent composition prepared by the process comprising: providing a first layer of wettable fibers; distributing on the first layer of wettable fibers a layer of surface crosslinked polymer having a FVAUL value of at least about 60 cc after 10 minutes; providing a second layer of wettable fibers on top of the layer of surface crosslinked polymer; and calendaring said layers to form the wettable fibers into a fibrous matrix having the surface crosslinked polymer disbursed therein, said surface crosslinked polymer comprising about 70% by weight to about 95% by weight the absorbent composition and said wettable fibers comprising about 5% by weight to about 30% by weight of the absorbent composition.
An even further embodiment of the present invention is an absorbent composition comprising: a surface crosslinked polymer having a FVAUL value of at least about 60 cc after 10 minutes.
An still further embodiment of the present invention is an absorbent article comprising: a permeable topsheet; a substantially impermeable backsheet; and an absorbent core disposed between the permeable topsheet and the substantially impermeable backsheet, said absorbent core comprising an absorbent composition comprising about 5% by weight to about 30% by weight of a fibrous matrix comprising wettable fibers and about 70% by weight to about 95% by weight of a surface crosslinked polymer having a FVAUL value of at least about 60 cc after 10 minutes, said surface crosslinked polymer being disbursed within said fibrous matrix.
Yet a further embodiment of the present invention is a method for preparing an absorbent composition comprising: providing a first layer of wettable fibers; distributing on the first layer of wettable fibers a layer of surface crosslinked polymer having a FVAUL value of at least about 60 cc after 10 minutes; providing a second layer of wettable fibers on top of the layer of surface crosslinked polymer; and calendaring said layers to form the wettable fibers into a fibrous matrix having the surface crosslinked polymer disbursed therein, said surface crosslinked polymer comprising about 70% by weight to about 95% by weight the absorbent composition and said wettable fibers comprising about 5% by weight to about 30% by weight of the absorbent composition.
These and other aspects of the invention will become apparent to those of ordinary skill in the art from the following description.